Hematopoietic stem cells are defined as those cells that are capable of both self-renewal and differentiation into the two principle precursor components--the myeloid and lymphoid lines. Such stem cells are said to be "totipotent." Stem cells that are less general but that can still differentiate into several lines are called "pluripotent." Further differentiation then occurs among the precursor cells to produce the monocyte, eosinophil, neutrophil, basophil, megakaryocytes, and erythroid lineages from the myeloid line, and T cells, B cells, and NK cells from the lymphoid line. For a background review of the stem cell see Scientific American 256:86-93 (December 1991).
One of the first breakthroughs into stem cell isolation and identification came in the late 1980's. In U.S. Pat. No. 4,714,680 (Dec. 22, 1987), Civin described a population of pluripotent lympho-hematopoictic cells that were substantially free of mature lymphoid and myeloid cells. Civin also described an antigen, MY-10, and a monoclonal antibody thereto, which was present on those cells. Those cells made up about 1% of all cells in normal adult bone marrow, and generally comprised a mixture of totipotent, and pluripotent stem cells and lineage committed precursor cells with the latter cells predominating.
Since that time, MY-10 has been classified by the International Workshop on Human Leukocyte Antigens as falling with the cluster designated as "CD34." Anti-CD34 monoclonal antibodies are now commercially available from a number of sources including, for example, Becton Dickinson Immunocytometry Systems ("BDIS").
Anti-CD34 monoclonal antibodies have been used for a number of purposes. Loken, Terstappen and their collaborators have published a series of papers describing the maturational stages for various components of the hematopoietic system, such as B lymphocytes (Loken et al., Blood 70:1316-1324 (November 1987)), erythroid cells (Loken et al., Blood 69:255-263 (January 1987)), and neutrophils (Terstappen et al., Leukemia 4:657-663 (September 1990)). The objective of these studies was to define, starting from the most mature cell and working backwards, the various maturational and developmental stages of a lineage committed cell.
Anti-CD34 monoclonal antibodies have also been used to look for earlier non-lineage committed stem cells. For example, Terstappen et al., Blood 77:1218-1227 (March 1991), described a subset of human progenitor cells that were capable of self-renewal and differentiation into each of the various hematopoietic lineages (i.e., a population of cells that include cells that are totipotent). This population was characterized as being CD34.sup.+ /CD38.sup.-.
U.S. Pat. No. 5,061,620 to Tsukamoto et al. (Oct. 29, 1991) also described a population of cells that were capable of self-renewal and differentiation. This population of cells was characterized as being CD34.sup.+ /CD10.sup.- /CD19.sup.- /CD33.sup.- and Thy-1.sup.+.
Other investigators have attempted to subset CD34.sup.+ cells from both peripheral blood and bone marrow. Bender et al., Blood 77:2591-2596 (June 1991), used four color flow cytometry with combinations of monoclonal antibodies (i.e., anti-CD34, anti-CD33, anti-CD45, anti-CD19, anti-CD7, anti-CD10, anti-CD3, anti-CD20, anti-CD14, anti-CD11b and anti-HLA-DR), to identify and isolate CD34.sup.+ hematopoietic progenitor cells. Bender et al. were able to identify a number of subsets. One subset was CD34.sup.+ /HLA-DR-. This subset had a very small number of cells and no clear population of this phenotype was resolved. Bender et al. speculated on the ability of this population of cells to give rise to blast cell colonies or cells reconstituting long term cultures based upon prior work of others.
Sutherland et al., Blood 78:666-672 (August 1991), reported on the differential regulation of "primitive" hematopoietic cells in long term culture. They used anti-CD34 and anti-HLA-DR monoclonal antibodies to select cells that were CD34.sup.+ and HLA-DR.sup.dim or HLA-DR.sup.-. These cells were then grown on a unique stromal cell line. The purpose of this work was to establish a method of long term culture of such cells for the purposes of studying hematopoiesis and the effect of different growth factors on hematopoiesis.
Simmons et al., Blood 78:55-62 (July 1991), also reported on the "identification" of a stromal cell precursor in human bone marrow. Using an antibody they designated "Stro-1," Simmons et al. were able to remove stromal cells from bone marrow. The antigen recognized by this antibody was not present on colony forming progenitor cells but was present on a "subpopulation of cells experiencing the [CD34] antigen." Thus, Simmons et al. described the ability of the antibody to separate out stromal cells from hematopoietic cells in bone marrow before culture.
Verfaillie et al., J. Exp. Med. 172:509-520 (August 1990), reported on a CD34.sup.+ /HLA-DR.sup.+ and CD34.sup.+ /HLA-DR.sup.- population of "primitive" progenitor cells. Taking adult marrow, Verfaille et al. depleted bone marrow of lineage.sup.+ cells using multiple monoclonal antibodies. Next, fluorescently labeled CD34 and HLA-DR monoclonal antibodies were used to select HLA-DR.sup.+ and HLA-DR.sup.- populations that were also CD34.sup.+. Having isolated these two groups, Verfaille et al. reported that the HLA-DR.sup.+ cells were better in short term culture than the HLA-DR.sup.- cells. In long term culture, the reverse was true.
WO93/25216, published Dec. 23, 1993, teaches a population of human primitive stem cells that are capable of self-renewal and that are capable of differentiating into hematopoietic stem cells and stromal stem cells that give rise to the hematopoietic microenvironment. This population of cells has the phenotype CD34.sup.+ /CD38/HLA-DR. This population of cells lacks lineage committed antigens (i.e., is CD33-, CD10-, CD5-, and CD71-). Cells having this phenotype were identified in adult and fetal peripheral blood, bone marrow, thymus, liver, or spleen using a combination of antibodies and selecting for the presence or absence of the antigens recognized by these antibodies on the cells. Preferably, the combination of antibodies comprised at least three monoclonal antibodies and more preferably comprised anti-CD34, anti-CD38 and anti-HLA-DR monoclonal antibodies.
WO94/02157, published Feb. 3, 1994, teaches the isolation of human hematopoietic stem cells that are CD34.sup.+, HLA-DR and express the receptor for the c-kit ligand (KR.sup.+). This cell population was reportedly useful for transplantation and in gene therapy protocols.
To date, the CD34 antigen, as identified by monoclonal antibodies, has been the only known cell surface marker to be used to define the hematopoietic stem cell compartment and has become the marker of choice not only for the identification of stem cells but also for their isolation. Published information now indicates the existence of monoclonal antibodies that define cell surface markers distinct from CD34; (i) monoclonal antibody AC133 which binds to a surface protein of 96 kDa on approximately 50% of CD34.sup.+ cells; (ii) monoclonal antibody BB9 which binds to a surface protein of 160 kDa on approximately 10-28% of CD34.sup.+ cells; and (iii) a non-designated monoclonal antibody that binds to a glycoprotein 105 on the surface of hematopoietic stem cells. Virtually all of the CFU-S and colony forming unit cells detectable by in vitro stem cell assays express the CD34 antigen. Furthermore, a number of animal and human studies have demonstrated that purified CD34.sup.+ cells are capable of reconstituting the entire hematopoietic system, suggesting that early engraftment by progenitor cells and long-term maintenance by primitive stem cells are mediated by this population (See, e.g., Berenson et al., J. Clin. Invest. 81:951-955 (1988)).
The identification and isolation of the most primitive population of hematopoietic stem cells would be highly advantageous in situations where reinfusion of only a small number of long-term repopulating cells was desired. For example, this would be the case when purging bone marrow or peripheral blood stem cells of contaminating tumor cells, or where genetic manipulation of the stem cells was the objective. CD34 expression seems to be stage specific rather than lineage specific with higher levels of expression seen in primitive progenitors and decreasing expression levels with cellular maturation (Holyoake & Alcom, Blood Rev. 8(2): 113-124 (1994)). Nonetheless, it has never been successfully demonstrated that stem cells could be purified on the basis of their CD34 expression levels. The studies described above suggest that CD34.sup.+ cells selected for the absence of lineage specific markers, such as, CD33, CD38, HLA-DR, as well as low Thy-1 labeling (Thy.sup.lo) (Craig et al., J. Exp. Med. 177:1331-1342 (1993)), correspond to a stem cell-enriched population. No positive enrichment procedure, however, has ever been described.
Clearly, there is a continuing need in the art to isolate novel markers of primitive stem cell populations so that positively enriched primitive stem cell populations can be obtained and utilized as therapeutic compositions, as well as in therapeutic methods such as bone marrow transplantation and gene therapy.
Bone marrow transplantation is an effective therapy for an increasing number of diseases. Graft Versus Host Disease (GVHD), however, limits bone marrow transplantation to recipients with HLA-matched sibling donors. Even then, approximately half of the allogenic bone marrow transplantation recipients develop GVHD. Current therapy for GVHD is imperfect and the disease can be disfiguring and/or lethal. Thus, risk of GVHD restricts the use of bone marrow transplantation to patients with otherwise fatal diseases, such as malignancies, severe aplastic anemia, and congenital immunodeficiency states.
The potential benefits from expanded use of bone marrow transplantation have stimulated research on the cause and prevention of GVHD. It has been shown that donor T lymphocytes cause GVHD in animals. Removal of T lymphocytes from donor marrow inocula ("grafts") prevented the subsequent development of GVHD in mice, dogs, and monkeys. Similar trials in humans with monoclonal antibodies against human T lymphocytes are now in progress. Preliminary results, however, suggest only attenuation of GVHD, not a cure. Similar results have been achieved with E-rosette and soybean lectin depletion of T lymphocytes. Another approach under investigation is the use of anti-T lymphocyte monoclonal antibodies conjugated to toxins, such as ricin.
As of yet, however, GVHD has not been prevented or cured in bone marrow recipients. Therefore, a continuing need exists for improved methods of combating Graft Versus Host Disease.
Donors of bone marrow are also faced with undesirable procedures and risks. The current procedures for harvesting bone marrow are expensive and painful. Furthermore, the current donation procedure is accompanied by the risks associated with anesthesia, analgesia, blood transfusion and possible infection. It would be desirable, therefore, to improve the current method of harvesting hematopoietic stem cells from donors.